1. Field of the Invention
The present invention relates to a semiconductor device with a semiconductor substrate.
2. Description of the Background Art
As a transistor made of semiconductor, which is an example of a semiconductor device with a semiconductor substrate, transistors of a DRAM (Dynamic Random Access Memory) are widely known. FIG. 5 is a sectional view of a conventional transistor of a DRAM. The conventional transistor of a DRAM depicted in FIG. 5 has the following structure.
In the conventional transistor, an SiGe crystal film substrate 2 containing a p type impurity is formed on a semiconductor substrate 1. Furthermore, an Si crystal film 4 containing a p type impurity, which functions as a channel region, is formed on SiGe crystal film substrate 2. In addition, on opposite sides of Si crystal film 4, an Si crystal film 3 containing an n type impurity, which functions as a source/drain region, is formed. Moreover, a gate electrode 6 is formed on Si crystal film 4 with a gate insulator film 5 interposed.
The aforementioned Si crystal film 3, Si crystal film 4, gate insulator film 5, and gate electrode 6 together form an n channel transistor 7.
In the conventional transistor described above, Si crystal film 4 is formed on SiGe crystal film substrate 2. Therefore, a lattice distortion is produced in Si crystal film 4. As a result, the mobility of electrons flowing in Si crystal film 4 is improved, i.e, the conventional transistor increases a drain current by utilizing Si crystal film 4 formed on SiGe crystal film substrate 2 as a material of a channel region.
It is noted that, in a conventional technique, Si crystal film 4 is described as an example of a conductive portion formed in a semiconductor substrate, in which electrons flow, and that SiGe crystal film 2 is described as an example of a mobility-improving portion contacting a lower surface of the conductive portion to produce a lattice distortion in semiconductor constituting the conductive portion to improve the mobility of electrons flowing in the conductive portion.
The conventional transistor described above has a structure in which a surface of Si crystal film 4 as a channel region contacts the gate insulator film. Thus, in the step of forming gate insulator film 5, the surface of Si crystal film 4 is exposed. Because of such a structure, a native oxide or the like tends to be formed on the surface of Si crystal film 4 as an example of a conductive portion. Consequently, after gate insulator film 5 is formed, the surface of Si crystal film 4 is oxidized because of the existence of gate insulator film 5. This results in a decrease in dielectric constant of gate insulator film 5 itself. This eventually makes a channel region formed in Si crystal film 4 incomplete, when a voltage is applied to the gate electrode. As a result, a problem arises that a desired drain current cannot be obtained.
Furthermore, in the conventional transistor, a surface of Si crystal film 3 as a source/drain region is in contact with a conductive member (e.g. contact plug) passing a current through the source/drain region. In this step of forming the conductive member as well, a surface of Si crystal film 3 is exposed. Because of such a structure, in the conventional transistor, a native oxide or the like tends to be formed on the surface of Si crystal film 3 as an example of a conductive portion, and this increases a contact resistance between a conductive member and Si crystal film 3 as a source/drain region. As a result, a problem arises that the operating speed of a transistor decreases.
An object of the present invention is to provide a semiconductor device that solves the problem caused by a decrease in conductivity in a conductive portion resulting from a native oxidation of a surface of a conductive portion.
Another object is to provide a semiconductor device that solves the problem caused by a decrease in conductivity in an Si crystal film resulting from a native oxidation of a surface of an Si crystal film.
In accordance with one aspect of the present invention, a semiconductor device includes a semiconductor substrate. The semiconductor substrate includes a conductive portion in which electrons flow, a mobility-improving portion contacting a lower surface of the conductive portion to produce a lattice distortion in semiconductor constituting the conductive portion to improve the mobility of electrons flowing in the conductive portion, and an oxidation-suppressing portion overlaying an upper surface of the conductive portion while constituting a surface of the semiconductor substrate to prevent native-oxidation of the conductive region.
In accordance with the configuration described above, the surface layer of the semiconductor substrate is made from the oxidation-suppressing portion less susceptible to native-oxidation, so that the problem resulting from the native oxidation of the surface layer of the semiconductor substrate is solved. In addition, electrons move in the conductive portion in which a lattice distortion is produced by the influence of the mobility-improving portion, so that the mobility of the electrons is improved.
In accordance with another aspect of the present invention, the semiconductor device includes a semiconductor substrate including an Si layer formed on a first SiGe layer, and a second SiGe layer formed on and in contact with the Si layer and constituting a surface layer, and the semiconductor device is utilized in a manner that electrons move in the Si layer.
In accordance with the configuration described above, the surface layer of the semiconductor substrate is made from a second SiGe layer less susceptible to native-oxidation, so that the problem resulting from the native oxidation of the surface layer of the semiconductor substrate is solved. In addition, electrons move in the Si layer in which a lattice distortion is produced by the influence of a first SiGe layer, so that the mobility of the electrons is not decreased. Furthermore, it is the first SiGe layer producing a lattice distortion in the Si layer that is formed on the surface of the Si layer, and therefore, decrease in the mobility of the electrons is less likely.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.